Photovoltaic cells (“PV cells”) convert light, such as sunlight, into an electric current. PV cells may provide electric power to a system using solely light as an energy source. Consequently, PV cells may provide electric power to a system without the use of traditional energy sources such as chemical energy sources (e.g., batteries, fossil fuels) and/or an electric power grid.
The ability of PV cells to convert light into an electric current makes PV cells an attractive electric power source in many applications wherein it is undesirable, impractical, uneconomical and/or impossible to use traditional energy sources. For example, PV cells commonly power emergency roadside call boxes located in rural areas where an electric power grid is not readily accessible. As another example, hand held calculators are commonly powered by one or more PV cells; the PV cells allow use of the calculator wherever light is present without requiring the use of batteries or a connection to an electrical outlet.
In certain applications, it is desirable that PV cells be at least partially transparent to one or more predetermined frequencies of radio frequency electromagnetic radiation. For example, if PV cells are to power a radar unit from sunlight, it may be desirable to place the PV cells on an outer surface of the radar unit so that the radar unit's structure will not shadow the PV cells. However, prior art PV cells are generally opaque to radio frequency electromagnetic radiation and therefore, may interfere with operation of the radar unit if placed on the radar unit's outer surface. This opaqueness is largely due to the fact that prior art PV cells generally contain at least one layer of metal, which blocks the passage of radio frequency electromagnetic radiation through the PV cell.
A common source of metal in a PV cell is the PV cell's back contact layer. The back contact layer is one of two electrical contact subsystems in the PV cell that enable it to be electrically connected to an external system. Stated in another manner, the back contact layer may serve as one of two electrical terminals on the PV cell, much like a terminal on a battery. The back contact layer functions to collect electrical charge from the back section of the PV cell. Consequently, the back contact layer must be an electrical conductor and must be in electrical contact with the PV cell.
As its name implies, the back contact layer is generally located near the back or bottom of the PV cell. The back of the PV cell is the PV cell's side that is opposite from the PV cell's light source; the front or top of the PV cell is the PV cell's side that is proximate to the light source. The PV cell's other electrical contact subsystem is often referred to as the top contact layer or subsystem and is commonly located near the front of the PV cell.
A back contact layer may be formed of a material that is non-transparent to light, such as molybdenum, aluminum or nickel. Such materials are also non-transparent to radio frequency electromagnetic radiation. A back contact layer also may be formed of a material that is transparent to light, such as zinc oxide or indium tin oxide. Although such materials are transparent to light, they are non-transparent to radio frequency electromagnetic radiation.
Many prior art PV cells have a solid metallic back contact layer. For example, a prior art PV cell may have a solid back contact layer formed of or containing molybdenum. Although such cells may exhibit relatively high efficiencies, the solid metallic back contact layer also blocks the passage of radio frequency electromagnetic radiation through the PV cell.
Other prior art PV cells have a non-solid metallic back contact layer, an example of which is disclosed in U.S. Pat. No. 4,487,989 to Wakefield et al., entitled “Contact for Solar Cell.” Wakefield discloses a PV cell with a back contact layer having a “checkerboard pattern” intended to permit the “passage of radiation through the cell which would otherwise decrease efficiency due to heat generation.” Although Wakefield's back contact layer apparently passes at least some infrared radiation, which may generate heat, the back contact layer is not transparent to radio frequency electromagnetic radiation of one or more predetermined frequencies.
Thus, there is a need for a PV cell having a back contact layer that is at least partially transparent to radio frequency electromagnetic radiation of one or more predetermined frequencies (“radio frequency transparent PV cell”).